(a) Field of the Invention
The present invention relates to a dry-etching method utilizing a photochemical reaction, and an apparatus therefor.
(b) Description of the Prior Art
IC patterns are recently being micronized, and some trial products with a minimum pattern size on the order of 1 to 2 .mu.m, or even submicrons have been developed. Plasma etching is indispensable for such micropatterning. In a conventional plasma etching apparatus, a reactive gas such as CF.sub.4 is supplied to a chamber having parallel plate electrodes, and 13.56-MHz RF power is supplied across the electrode having a wafer thereon. A glow discharge occurs between the electrodes to generate a plasma. Positive ions in the plasma are accelerated by a cathode voltage drop, and the accelerated positive ions bombard the surface of the wafer to perform plasma etching. This etching technique is called reactive ion etching (RIE) and plays a major role in state-of-the-art micropatterning.
According to this technique, however, since the wafer to be etched is placed in the plasma, the following radiation damage occurs: the oxide film is damaged by charged particles such as ions and electrons, a threshold voltage of the resultant element is shifted by a soft X-ray, traps are formed in the oxide film, metal impurities from the inner wall surface of the chamber are introduced, and the like. Such radiation damage often results in decisive failures of VLSIs. Demand has arisen for developing an etching technique free of radiation damage.
Many conventional etching techniques, free of radiation damage, have been reported. Typical examples are anisotropic etching of Si or poly-Si by an atomic F beam having kinetic energy generated by a gas temperature in the glow discharge (e.g., H. Akiya, Proc. 3rd. Symp. on Dry Processes, P. 119 (1981)), and etching using laser or ultraviolet light (e.g., T. J. Chuang, J. Chem. Phys. 74, 1453 (1981); and H. Okano, T. Yamazaki, M. Sekine and Y. Horiike, Proc. of 4th Symp. on Dry Processes, P. 6 (1982)). Thus, possible non-damaging anisotropic etching techniques have been proposed.
According to extensive studies of the present inventors, it was found that the same effect as in ion-assist etching (e.g., J. W. Coburn and H. F. Winters, J. Appl. Phys. 50, 3189 (1979)) was obtained by poly-Si etching in a Cl.sub.2 atmosphere upon radiation of an ultraviolet ray emitted from an Hg-Xe lamp (e.g., H. Okano, T. Yamazaki, M. Sekine and Y. Horiike, Proc. of 4th Symp. on Dry Processes, P. 6 (1982)). In other words, the etching reaction on the irradiated surface was greater than that of a non-irradiated surface. This effect is typically observed in undoped poly-Si, single crystal Si and p-type boron-doped Si. In addition, such an effect can also be observed in n.sup.+ -type poly-Si, or in Mo, W, Ta or their silicides.
However, according to the above method, the following problem is presented. Reactive gas radicals dissociated in the gas phase propagate under a mask, and light is slightly scattered from the etching surface. In this case, the region under the mask is etched, i.e., an undercut occurs. In particular, when the etching mask comprises a photoresist mask, the mask is transparent to light, and light irradiates under the resist mask. As a result, an undercut tends to form. The undercut is the major cause preventing micropatterning of the element, resulting in a decisive drawback for VLSIs.
The following recent RIE studies have been reported. Side etching is prevented (e.g., C. J. Mogab and H. J. Levinstein, J. Vac. Sci. Technol. 17, 721 (1980)) by rebonding Cl radicals as an etchant and CF.sub.4 radicals, derived from C.sub.2 F.sub.6, as an additive gas. Various unsaturated monomers such as decomposites of the resist and discharge products are attached to the etched wall surface to prevent bombardment by the etchant (e.g., R. H. Bruce and G. P. Malafsky, E.C.S. Meeting, Abs. No. 288, Denver, (1981); or T. Yamazaki, H. Okano and Y. Horiike, 30th Appl. Phys. Prec., Spring Committee, 1983).